Mine eats every Saturday, poops every Sunday, and sits in his hide nonstop until he gets fed again or I pull him out. It's like clockwork.
He doesn't ever LEAVE his hide, but 90% of the time I come in to check on him, he's got his head poking out to see what's going on.
As for dehydration, if he's got a constant clean water supply, he should be okay. Ramses usually gets wrinkly skin around his curves when he's about to shed too, and then he plumps up again once it's done.